1. Field of the Invention
The present invention relates to a prepit detecting apparatus arranged to detect prepit information in an optical recording/reproducing apparatus for recording/reproducing information with respect to a recording-enabled optical recording medium having pre-information, such as address information, in the form of prepits.
2. Description of the Related Art
In general, a recording-enabled optical recording medium is enabled to record information on an optical recording medium on which no information has been recorded by recording address information and reference signals for generating a clock signal for use in a recording/reproducing operation in the form of prepits and pregrooves. A DVD-R (Digital Versatile Disc-Recordable) has energetically been researched and developed, the DVD-R being a recording medium which is capable of recording information in a quantity which is about seven times information which can be recorded on the CD. The DVD-R has pregrooves which are regions on which information, such as video data and audio data, which must be recorded is recorded as recording pits. Moreover, the DVD-R has prepits (hereinafter called land prepits (LPP)) in land portions which are regions formed among the foregoing pregrooves. The LPP is formed on a straight line which perpendicularly intersect the direction of a tangent of the pregrooves such that the land prepits are not formed adjacent to each other across a pregrooves. The pregrooves are slightly swung (wobbled) in the radial direction of the disc at a frequency based on a reference clock which is used to control the rotation of the DVD-R. When the rotation of the DVD-R is controlled, the wobbling frequency is detected. Thus, feedback control is performed in such a manner that the detected wobbling frequency coincides with the frequency of the reference clock.
The LPP is detected such that reflected light of a light beam with which the pregrooves has been irradiated is received by a light receiving device divided into two sections at least along a division line optically parallel with the direction of a tangent of the pregrooves. Moreover, the difference of output signals from the regions (the divided regions) of the light receiving device in a direction perpendicular to the pregrooves is calculated. The obtained differential signal is compared with a predetermined threshold so that the LPP is detected as a binary signal (hereinafter called an xe2x80x9cLPP signalxe2x80x9d). When the recording medium is an optical disc, the divided regions of the light receiving device are formed in the radial direction of the disc owing to the foregoing division line. Therefore, the foregoing differential signal is called a xe2x80x9cradial push-pull signalxe2x80x9d.
The reason why the LPP can be detected by using the radial push-pull signal is that the LPP is formed as described above such that the level does not exist in adjacent land portions on a straight line which perpendicularly intersects the direction of tangent of the pregroove. That is, when one pregroove is irradiated with a light beam, reflection components of the LPP do not simultaneously exist in light reflected from the two side land portions (the LPP component exists in only reflected light from either of the land portions). Thus, the foregoing calculation for obtaining the difference enables only the component of light reflected from the LPP to be extracted. Usually, only either (for example, the positive pole component) of the two polar components obtained by the calculation for obtaining the difference is compared with the predetermined threshold. An obtained binary signal is employed as the LPP signal.
A record-enabled optical recording medium, such as the DVD-R, encounters reduction in the reflectance owing to thermal energy of a recording beam when a high output light beam is applied to form the recording pit for storing information. That is, the quantity of light reflected from the position of a pregroove in which the recording pit has been formed (which has been irradiated with the recording beam) is smaller than the quantity of light reflected from the position of a pregroove having no recording pit. If the recording beam is also used as the reproducing beam (when the recording beam is used as the reproducing beam, a low output is employed) or if a tracking error signal can be generated by one beam, the structure of a recording/reproducing apparatus can conveniently be simplified. Therefore, the diameter of the recording beam is usually adjusted to be slightly larger than the width of the pregroove. Thus, when information is recorded, also a portion of the land adjacent to the position of the pregroove on which the recording pit for storing information is formed is irradiated with the recording beam. Therefore, if an LPP exists in the land portion adjacent to the position of the pregroove on which the recording pit is formed, the following problem arises when the LPP is read after the recording pit has been formed.
That is, irradiation with the high-output recording beam for the purpose of forming the recording pit causes the reflectance of the land portion having the LPP to be reduced. Therefore, when the LPP is read, the quantity of light reflected from the LPP is reduced. Also the amplitude level of the differential signal of the LPP which is extracted as the radial push-pull signal is lowered. As a result, there arises a problem in that the S/N ratio (signal-to-noise ratio) of the LPP signal with respect to unnecessary noise deteriorates.
On the other hand, the pregrooves are wobbled in the radial direction of the disc as described above. Therefore, the radial push-pull signal is formed into a composite signal in which the prepit is superimposed on the component of the wobbling frequency. A disc, such as the DVD-R, on which information has densely been recorded, sometimes encounters a fact that the component of the wobble signal of a pregroove adjacent to a pregroove which is irradiated with a light beam is leaked and introduced owing to crosstalk. If the foregoing leakage and introduction occur, the component of the wobble signal in the foregoing composite signal is undesirably caused to interfere. Thus, the amplitude is undesirably changed.
That is, the component of the LPP signal is superimposed on the wobble signal having the amplitude which is changed. Since the amplitude of the wobble signal serving as the base-line voltage is undesirably changed, comparison with a fixed slice level for detecting the LPP signal in the form of a binary signal cannot easily be performed.
To overcome the interference of the wobble signal component, for example, a method exists with which the amplitude of the wobble is AM-detected to obtain the amplitude change component. Moreover, the obtained change component is reduced to a slice level for binary-coding the LPP, followed by performing binary coding while following to change in the wobble is being performed.
However, when the foregoing method is employed, an AM wave detecting circuit, a variety of filters and suppression of the LPP component when the wobble amplitude is detected are required. Therefore, the size of the circuit is enlarged excessively. Moreover, setting and adjustment of a quantity of injection of the change in the wobble must be performed. Therefore, there arises a problem in that the operation for adjusting the circuit becomes complicated.
In view of the foregoing, an object of the present invention is to provide a prepit detecting apparatus having a simple structure and capable of accurately extracting an LPP signal.
To solve the problem, according to one aspect of the present invention, there is provided a prepit detecting apparatus which uses an optical recording medium having information recording tracks on which information is recorded and guide tracks each of which guides a light beam to the information recording track and on which prepits for saving pre-information are formed such that the light beam is applied to the information track of the optical recording medium, the prepit detecting apparatus comprising: light receiving means divided into a first divided light receiving portion and a second divided light receiving portion divided along an optically parallel division line in a direction of tangent of the information tracks and arranged to receive reflected light of the light beam with which the optical recording medium has been irradiated; and a difference calculator for calculating the difference between a first read signal output from the first divided light receiving portion and a second read signal output from the second divided light receiving portion so that the prepit is detected in response to a differential signal output from the difference calculator, wherein the difference calculator incorporates amplitude correction means for causing the amplitudes of the first read signal and the second read signal to coincide with a reference level, and the difference between the first read signal and the second read signal corrected by the amplitude correction means is calculated.
Another aspect of the present invention has a structure that the amplitude correction means incorporates first amplitude extracting means for extracting an average amplitude level of the first read signal, second amplitude extracting means for extracting an average amplitude level of the second read signal, first comparison means for comparing an output of the first amplitude extracting means and an output of the second amplitude extracting means with each other, second comparison means for comparing an output of the second amplitude extracting means and the reference level with each other, first amplitude adjustment means for adjusting the amplitude of the first read signal in accordance with an output of the first comparison means and second adjustment means for adjusting the amplitude of the second read signal in accordance with an output of the second comparison means.
Another aspect of the present invention has a structure that the amplitude correction means incorporates first amplitude extracting means for extracting an average amplitude level of the first read signal, second amplitude extracting means for extracting an average amplitude level of the second read signal, first comparison means for comparing an output of the first amplitude extracting means with a reference level, second comparison means for comparing an output of the second amplitude extracting means with a reference level, first amplitude adjustment means for adjusting the amplitude of the first read signal in accordance with an output of the first comparison means and second adjustment means for adjusting the amplitude of the second read signal in accordance with an output of the second comparison means.
Since the recording pits for storing data are formed, the amplitude correction means corrects reduction in the amplitude of each of the first and second read signals if the quantity of light reflected from the LPP which must be extracted is reduced. Therefore, the LPP signal can accurately be extracted.
According to another aspect of the present invention, there is provided a prepit detecting apparatus arranged to use an optical recording medium which has information recording tracks on which information is recorded, which is wobbled in response to a wobble signal having a predetermined frequency and which has guide tracks for guiding a light beam to the information recording track having prepits for saving pre-information and incorporating light receiving means divided into a first divided light receiving portion and a second divided light receiving portion divided along an optically parallel division line in a direction of tangent of the information tracks and arranged to receive reflected light of the light beam with which the optical recording medium has been irradiated and a difference calculator for calculating the difference between a first read signal output from the first divided light receiving portion and a second read signal output from the second divided light receiving portion so that the prepit is detected in response to a differential signal output from the difference calculator, the prepit detecting apparatus comprising: DC clamping means for clamping the differential signal output from the difference calculator with a DC level; and prepit detecting means which compares an output from the clamping means and a reference slice level with each other to detect the prepit signal, wherein the DC clamping means clamps the wobble signal component corresponding to detection timing of the prepit signal detected by the prepit detecting means to the predetermined DC level.
Another aspect of the present invention has a structure that the DC clamping means incorporates relay means for relaying the differential signal at detection timing of the prepit signal, an integrating circuit for integrating the differential signal supplied through the relay means and superimposing means for superimposing an output of the integrating circuit on the differential signal, and the integrating circuit incorporates a calculation amplifier having a non-inverted input terminal to which a predetermined DC level is input and an integrating capacitor connected between an output terminal of the calculation amplifier and an inverted input terminal.
Therefore, if the base voltage of the first transition of the LPP signal is changed, the LPP can accurately be extracted.